1. Field of the Invention
The present invention relates to a working method for forming a tube into a desired shape with predetermined dimensions through a plastic working and a working apparatus suitable for carrying out the method.
2. Description of Related Art
As a typical apparatus that uses a tube, there is a hydraulic shock absorber, and such hydraulic shock absorber is known, for example as shown in FIG. 8 and FIG. 9 wherein it comprises an inner tube 2 accommodated in a bottomed outer tube 3 to receive a piston 1 for sliding, a rod 4 connected at its one end with the piston 1 and extended outwardly at its opposite end passing through a rod guide 5 engaged jointly with the opened ends of the inner and outer tubes 2 and 3, piston valves 7 provided on the piston 1, and a base valve 8 provided at the inner bottom of the outer tube 3. These piston and base valves serve to create damping forces during strokes of the piston-rod in extension and compression directions, allowing hydraulic fluid filled in the inner tube 2 to pass through these valves. The volume of the hydraulic fluid corresponding to advancement or retraction of the piston rod 4 may be compensated for by means of a reservoir 9 in which gas and hydraulic fluid are filled between inner and outer tubes 2 and 3.
In this type of hydraulic shock absorber, a rod guide 5 and an oil seal 6 are press-fitted on the outer tube 3 at its opened end. Then, the rod guide 5 and the oil seal 6 are adapted to resist their withdrawal by means of a bent piece 10 which has been formed by bending this portion inwardly over the entire circumference of the opened end of the outer tube 3 or partially bent at a plural points in the circumferential direction. (see Japanese Unexamined Utility Model Application Publication No. HEI. 3-68650). A cap 11 that receives a bump rubber (not shown) is usually press-fitted on the end of the outer tube and fixed in position. The cap 11 is positioned in place by allowing a predetermined number (e.g., three) of protrusions arranged at the inner bottom side of the cap 11 so as to abut against the bent piece 10. In the meantime, there are also arranged an eye 12 serving as a mounting point at a vehicle side, a mounting member 13 serving as a mounting point at an axle side, and a spring receiver 14 for receiving a coil spring.
Because the opened end of the outer tube 3, which forms a part of the aforementioned hydraulic shock absorber, is formed such that its inner diameter surface is used as an engagement portion with the rod guide, and its outer diameter surface is used as a press-fitting section of the cap 11, a high degree of precision is required not only in working the inner and outer diameter surfaces, but also in assuring concentricity and circularity. Additionally, since the inner surface of the opened end of the outer tube 3 is also provided as an engagement portion with an oil seal 6 as previously described, an excellent surface smoothness must be ensured in installing the oil seal 6 so as not to impair the oil seal. Therefore, it has been a conventional practice to ensure a predetermined precision in dimensions, and shape and surface smoothness by machining or turning the tube end, using an electrically welded tube such as an electrically sewn-welded tube etc., as a blank tube.
Some attempts therefore have also been made to squeeze an end of the electric welded tube (blank tube) through a swaging process (rotary swaging process) for ensuring predetermined dimensions and shape (see, for example, the Japanese Unexamined Patent Application Publication No. HEI. 7-265979).
However, in the conventional approach of working the tube end through a machining operation which requires precise working of the tube, there is a problem that the machining operation itself involves an increased number of steps and extended working time which unavoidably create increased working costs. There also arises another problem in that cutting chips and burrs that may be produced during a machining operation may adhere to the tube's inner surface, and ingress into the hydraulic shock absorber as foreign materials (as contaminant).
Moreover, in the method for swaging the end of an electric welded tube, weld beads (welds) may remain on the electric welded tube, requiring a finishing working to be made by a turning operation, and thus it is far from proving a radical solution. In the meantime, the invention as described in the Japanese Patent Unexamined Application Publication No. HEI7-265979 contemplates an approach by which a female thread is formed on the inner surface of the tube end after the swaging operation is performed, and thus when such a post-working process is applied, it does not matter if there is a residue of beads. If a seamless tube is used as the blank tube as previously described, the need for applying a finishing operation may be eliminated, but in this case, an expensive seamless tube, unavoidably results in increased cost.